1. Field of the Invention
The present invention relates an image heating apparatus configured to heat an image formed on a recording sheet.
2. Description of the Related Art
There has been known a configuration adopting an electro-photographic or electrostatic recording method for an image forming apparatus configured to form an image on a recording sheet such as a cut sheet cut into a predetermined size. Such image forming apparatus is configured to form a toner image on a recording sheet by an image forming portion and to fix the toner image on the recording sheet by a fixing apparatus, i.e., an image heating apparatus. Glossiness of an image is often adjusted by passing a recording sheet on which an image has been formed through the fixing apparatus.
That is, the fixing apparatus of this type fixes the toner image on the recording sheet by rotating a fixing roller and a pressure roller in pressure contact with each other and by passing the recording sheet on which the non-fixed toner image has been formed through a nip portion formed by the fixing roller and the pressure roller whose temperatures are kept at predetermined temperatures. Still further, in order to prevent wrinkles of the recording sheet that are otherwise prone to be generated in passing through the nip portion, there is known a configuration in which at least one of the fixing roller and the pressure roller is formed into a so-called an ‘inverted crown’ shape such that outer diameters at both ends in a longitudinal direction thereof are maximized and an outer diameter of a center portion thereof is minimized.
However, this configuration forcibly stretches the recording sheet in the longitudinal direction of the fixing roller and others. Accordingly, when a limp recording sheet such as a thin sheet is passed through the nip portion, the strain exerts on both ends of the recording sheet, so that the both ends of the recording sheet undulate, causing a so-call “waviness” and degrading quality of the recording sheet.
Then, Japanese Patent Application Laid-open No. H2-262684 has proposed a configuration of giving firmness on both end portions of a recording sheet by positioning a maximum outer diameter portion of a pressure roller configured into the inverted crown shape at an inner part, rather than both end portions thereof, to form a slight line on part of the recording sheet that comes into contact with the maximum outer diameter portion of the pressure roller.
Japanese Patent Application Laid-open No. H9-179435 has proposed a configuration of counting a number of recording sheets fed through a fixing apparatus and of reducing a pressurizing force of a pressure roller applied to a fixing roller corresponding to a counted value. That is, when the recording sheets pass successively through a nip portion between the fixing roller and the pressure roller, diameters of the fixing roller and the pressure roller vary in a longitudinal direction thereof due to an increase of temperature of non-sheet fed part(s) of the nip portion where the recording sheets do not contact in passing through the nip portion, so that a drum effect of varying distribution of pressure of the nip portion increases. In this case, it is possible to relax the drum effect and to keep the distribution of the pressure of the nip portion in an adequate condition by reducing the pressure as described above. As a result, it is possible to prevent the waviness of the recording sheet and to smoothly convey the recording sheet.
Lately, the image forming apparatus is digitized more and more and is made possible to form an image by processing a large amount of information. Also a need of variable printing of outputting information of each user on printed publications is increasing. Then, when the image forming apparatus is used as a variable printing machine, the same level of quality of sheets with that of a printing machine is naturally requested, so that it is required to improve quality of sheets of products of the image forming apparatus more than ever before.
Here, the sheet, i.e., the recording sheet, has grain of fibers in making the sheet of paper. The recording sheet has such a nature that tensile strength thereof is strong in a direction of the grain and is weak in a direction orthogonal to the direction of the grain. Accordingly, when a conveying direction of the recording sheet conveyed through the nip portion of the fixing apparatus is in parallel with the direction of the grain (long grain direction), the recording sheet is prone to deflect in a direction orthogonal to the conveying direction, so that sheet wrinkles are prone to be generated. Meanwhile, when the conveying direction of the recording sheet is orthogonal to the direction of the grain (short grain direction), the recording sheet is prone to deflect in a direction in parallel with the conveying direction, so that waviness is prone to be generated.
Meanwhile, it is a general practice in the field of the variable printing to print materials by determining grain of sheets beforehand depending on what kind of products is to be produced. That is, when a book is to be made by two-folding large size sheets, short grain sheets are used so that the sheets are hardly torn when pages of the sheets are turned for example. When a sheet, e.g., a poster, is not folded, a long grain sheet is used so that the sheet is hardly rounded.
It is then difficult to obtain such products that fully accommodate to user's desire by the abovementioned configurations described in Japanese Patent Application Laid-open Nos. H2-262684 and H9-179435. That is, desired products may not be obtained if a fixing condition of the fixing apparatus is uniformed regardless of grains of sheets and of types of products to be produced.